Miniature display devices are useful in applications such as portable displays for video simulation applications, among others. A miniature display in the context of this discussion is understood to be a display sufficiently small that it requires an optical magnification arrangement to be effective. An advantage of such a display is that it consumes less power than a conventional display having real dimensions equal to the apparent dimensions of the magnified miniature display.
A particularly effective device for use as a spatial light modulating component in a miniature display system is a reflective grating light-valve (GLV) array. Such displays are described in detail in U.S. Pat. No. 5,459,610. This type of reflective grating light-valve array is capable of providing displays of very high resolution, very high switching speeds and high bandwidth by virtue a the very small size (about 1.times.40 micrometers) of operable elements of the array. The very small operable elements can be operated electrostatically with low applied voltage, such that, in combination with diode laser illumination and appropriate optics, it is potentially feasible to build a palm-sized projection display powered by (Q) dry cell batteries.
A significant problem in designing such a display system arises from the fact that the GLV array modulates light by diffraction, and light incident on the array for modulation is returned as a combination of reflected and diffracted beams. Because of this, an optical system used with the display must be capable, not only of magnifying, focussing or projecting an image of the GLV array to form a displayed image, but must also be capable of separating the diffracted light from the reflected light.
A most optical arrangement for providing separation of the diffracted and reflected light is known as Schlieren optics. Schlieren optics make use of the fact that light which is diffracted from a GLV array leaves the GLV array at a different angle from light which is reflected from the array. The light may be diffracted at different angle depending on the diffraction order. Typically, the first (brightest) order is used for forming a displayed image. The schlieren optics system can be arranged such that at certain points in the system for example at pupil positions, the diffracted and reflected rays can be physically separated. This allows the reflected light to be intercepted by a stop, thereby permitting, in theory at least, only the diffracted light to pass the stop for providing the display image.
There are several problems inherent in a schlieren optics system. For example, the requirement for separating diffracted light makes illumination of the GLV array for providing the diffracted light somewhat inefficient. It is also difficult to make a stop one-hundred percent effective in intercepting the reflected light, because of practical limitations (aberrations) of the optics. Any reflected which passes the stop is, in effect, stray light, which has the effect of reducing image contrast. Stray light can also be contributed by ghost reflections from optical surfaces. These ghost reflections are not direct by the optical system towards the stop. Problems in separating reflected and diffracted light are also created by dispersion of diffracted light and by unwanted diffracted orders.
There is an need for an alternate approach to forming a display using a GLV device. The approach should not rely on physical separation of diffracted and reflected light from the GLV array for forming an image.